Procedures in organic syntheses frequently employ a step in which a alkyl moiety that is pendent on an aromatic ring is oxidized. Typically, the alkyl moiety is oxidized to the corresponding carboxylic acid moiety or alcohol moiety. However, when the alkyl moiety is methyl, attempts at selective oxidation to convert the methyl moiety into something other than the corresponding carboxylic acid moiety or hydroxymethyl moiety have been extremely difficult.
For instance, it is well known to those skilled in the art that the methyl moiety in toluene can be selectively oxidized in order to convert the toluene into benzaldehyde using a lengthy 2-step procedure that requires formation of an intermediate. First, the methyl moiety is chlorinated with heat (i.e., chlorine gas is bubbled into boiling toluene) to form benzal chloride as the intermediate. This first step is followed by hydrolysis at an elevated temperature to remove the 2 chlorines from the dichloromethyl moiety and result in the formation of HCl and benzaldehyde. See, Morrison and Boyd, Organic Chemistry, 2d Ed., p. 386 and p. 619 (1966).
Additionally, as reported by Mijs et al. in Organic Synthesis by Oxidation with Metal Compounds (1986), transition metal oxidants have been used to attack the pendent methyl moiety in toluene. Nevertheless, the reaction is difficult to stop at the benzaldehyde stage and readily produces a high yield of benzoic acid.
To stop the benzaldehyde from further oxidizing to the corresponding benzoic acid, two main procedures are employed instead of the transition metal oxidant treatment.
In the first procedure, benzeneseleninic acid is employed to achieve selective oxidation of toluene to benzaldehyde, as reported by Barton et al. in "Preparation of Aldehydes and Ketones by Oxidation of Benzylic Hydrocarbons with Benzeneseleninic Anhydride", Vol. 35, Tetrahedron Letters, pp. 3331-3334 (1979). However, not only are yields poor, but also reaction times are long.
The second procedure employs glacial acetic acid or acetic anhydride to trap p-nitrobenzaldehyde as p-nitrobenzylidene diacetate, and then, the trapped intermediate is converted to p-nitrobenzaldehyde using chromic acid and chromyl acetate as oxidants (the procedure is substantially similar for the o-isomer), as reported by Nishimura, in Vol. 4, Organic Synthesis, pp. 713-715 (1963).
As is well known, aromatic aldehydes and substituted aromatic aldehydes are commercially important. For instance, benzaldehyde is used in the manufacture of dyes, in perfumery, and as a solvent, and nitrobenzaldehyde (a substituted benzaldehyde) is useful as a reagent for isopropyl alcohol and acetone.
Thus, it is desirable to find a way, unlike the above-described procedures, to achieve selective oxidation of toluene (or other aromatics having a pendent methyl moiety) into benzaldehyde (or other respective corresponding aromatics having a pendent aldehyde moiety) not only that can be carried out relatively quickly under relatively mild conditions but also that can achieve high yields of the resultant aldehyde.